Microphones with integral analog-to-digital converters, or digital microphones, are known in the art. EP 1 052880, WO 02/062101, U.S. Pat. No. 5,769,848 and GB 2319922 discloses several digital microphones for utilization in diverse applications such as professional audio, hearing instruments and mobile phones. WO 02/062101 discloses a microphone assembly comprising an electro-acoustical transducer coupled to a preamplifier. An amplified signal is coupled to an analog-to-digital converter which in one embodiment comprises a single-bit delta-sigma modulator. A disclosed embodiment of the microphone assembly comprises a formatting circuit that converts signal samples generated by the delta-sigma modulator into digital signals in accordance with a standardized digital audio transmission protocol such as S/PDIF, I2S or AES/EBU.
U.S. Pat. No. 6,326,912 discloses an analog-to-digital converter comprising a front-end multi-bit delta-sigma modulator coupled directly, or indirectly, to a back-end single-bit delta-sigma modulator for professional audio applications such as Super Audio Compact Discs that are based on a bit stream format or DVD Audio Discs that are based on a 24-bit PCM format.
While single-bit sigma-delta modulators have been successfully incorporated in a commercially available miniature microphone for hearing aid applications, there is a need for digital microphones employing integral analog-to-digital converters of improved performance. The improved digital microphones could advantageously be backward compatible with existing single-bit output devices. Improved performance analog-to-digital converters can be obtained by replacing conventional single-bit, or dual-level, quantizers with a multi-level quantizer in sigma-delta converter architectures.